In square LED track light fixtures in the prior art, a lens fixing plate and a light fixture assembly are connected to each other, generally fixedly connected by screws or by elastic sheets sometimes. Connecting the lens fixing plate with the light fixture assembly by screws or elastic sheets causes inconvenience in mounting and dismounting. As a result, the lens fixing plate and the light fixture assembly must be mounted or dismounted by tools or by a large force. Connecting the lens fixing plate with the light fixture assembly by screws or elastic sheets cannot play a role of light compensation to the light fixtures, that is, a lens mounted on the lens fixing plate cannot be in close contact with an LED light source mounted on an LED light source mounting plate of the light fixture assembly, resulting in low luminous efficiency of the light fixtures. With the continuous development of society and the continuous progress of science and technology, LED light fixtures are constantly upgrading, and people put forward new and higher requirements to the LED light fixtures in the prior art. Therefore, there is an urgent need to develop a connecting device for light fixtures, which is simple in structure and convenient in mounting and dismounting, has good light compensation, and enables the light fixtures to achieve maximum luminous efficiency.